The development of novel herbicide resistance in plants offers significant production and economic advantages. Rice production is frequently restricted by the prevalence of a weedy relative of rice that flourishes in commercial rice fields. The weed is commonly called “red rice,” and belongs to the same species as cultivated rice (Oryza sativa L.). The genetic similarity of red rice and commercial rice has made herbicidal control of red rice difficult. The herbicides Ordram™ (molinate: S-ethyl hexahydro-1-H-azepine-1-carbothioate) and Bolero™ (thiobencarb: S-[(4-chlorophenyl)methyl]diethylcarbamothioate) offer partial suppression of red rice, but no herbicide that actually controls red rice is currently used in commercial rice fields because of the simultaneous sensitivity of existing commercial cultivars of rice to such herbicides. The development of mutant commercial rice lines resistant to herbicides that are effective on red rice will greatly increase the ability to control red rice infestations.
Rice producers in the southern United States typically rotate rice crops with soybeans to help control red rice infestations. While this rotation is not usually desirable economically, it is frequently necessary because no herbicide is currently available to control red rice infestations selectively in commercial rice crops. During the soybean rotation, the producer has a broad range of available herbicides that may be used on red rice, so that rice may again be grown the following year. United States rice producers can lose $200–$300 per acre per year growing soybeans instead of rice, a potential loss affecting about 2.5 million acres annually. Additional losses in the United States estimated at $50 million per year result from the lower price paid by mills for grain shipments contaminated with red rice. Total economic losses due to red rice in southern United States rice production are estimated to be $500 to $750 million a year. Economic losses due to red rice are even greater in other rice producing countries.
Rice producers typically use the herbicides propanil (trade name Stam™) or molinate (trade name Ordram™) to control weeds in rice production. Propanil has no residual activity. Molinate is toxic to fish. Neither of these herbicides controls red rice. Imazethapyr ((±)-2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-5-ethyl-3-pyridinecarboxylic acid) offers an environmentally acceptable alternative to molinate, has the residual weed control activity that propanil lacks, and is a very effective herbicide on red rice. Imazethapyr also offers excellent control of other weeds important in rice production, including barnyardgrass. Barnyardgrass is a major weed in rice production, and is currently controlled with propanil or molinate. However, there are reports that barnyardgrass is developing resistance to propanil.
The total potential market for rice varieties that are resistant to a herbicide that can control red rice is about 5.3 million acres in the United States, and the potential market outside the United States is much larger. World rice production occupies about 350 million acres. Red rice is a weed pest in rice production in the United States, Brazil, Australia, Spain, Italy, North Korea, South Korea, Philippines, Vietnam, China, Brazil, Argentina, Colombia, India, Pakistan, Bangladesh, Japan, Ecuador, Mexico, Cuba, Malaysia, Thailand, Indonesia, Sri Lanka, Venezuela, Myanmar, Nigeria, Uruguay, Peru, and in other rice-producing countries.
Herbicides that inhibit the enzyme acetohydroxyacid synthase would offer a number of advantages over currently available herbicides if they could be used in commercial rice production. Potential advantages include long residual activity against weeds, effective control of the more important weeds in rice production, including red rice, and relative environmental acceptability. Even in regions where red rice is not currently a problem, the availability of herbicide-resistant rice can have a major influence on rice production practices by providing the farmer with a new arsenal of herbicides suitable for use in rice fields.
U.S. Pat. No. 4,761,373 describes the development of mutant herbicide-resistant maize plants through exposing tissue cultures to herbicide. The mutant maize plants were said to have an altered enzyme, namely acetohydroxyacid synthase, that conferred resistance to certain imidazolinone and sulfonamide herbicides. See also U.S. Pat. Nos. 5,304,732, 5,331,107, and 5,718,079; and European Patent Application 0 154 204 A2.
Lee et al., “The Molecular Basis of Sulfonylurea Herbicide Resistance in Tobacco,” The EMBO J., vol. 7, no. 5, pp. 1241–1248 (1988), describe the isolation and characterization from Nicotiana tabacum of mutant genes specifying herbicide resistant forms of acetolactate synthase (also known as acetohydroxyacid synthase), and the reintroduction of those genes into sensitive lines of tobacco.
Saxena et al., “Herbicide Resistance in Datura innoxia,” Plant Physiol., vol. 86, pp. 863–867 (1988) describe several Datura innoxia lines resistant to sulfonylurea herbicides, some of which were also found to be cross-resistant to imidazolinone herbicides.
Mazur et al., “Isolation and Characterization of Plant Genes Coding for Acetolactate Synthase, the Target Enzyme for Two Classes of Herbicides,” Plant Physiol. vol. 85, pp. 1110–1117 (1987), discuss investigations into the degree of homology among acetolactate synthases from different species.
U.S. Pat. No. 5,767,366 discloses transformed plants with genetically engineered imidazolinone resistance, conferred through a gene cloned from a plant such as a mutated Arabidopsis thaliana. See also a related paper, Sathasivan et al., “Nucleotide Sequence of a Mutant Acetolactate Synthase Gene from an Imidazolinone-resistant Arabidopsis thaliana var. Columbia,” Nucleic Acids Research vol. 18, no. 8, p. 2188 (1990).
Examples of herbicide-resistant AHAS enzymes in plants other than rice are disclosed in U.S. Pat. No. 5,013,659; K. Newhouse et al., “Mutations in corn (Zea mays L.) Conferring Resistance to Imidazolinone Herbicides,” Theor. Appl. Genet., vol. 83, pp. 65–70 (1991); K. Sathasivan et al., “Molecular Basis of Imidazolinone Herbicide Resistance in Arabidopsis thaliana var Columbia,” Plant Physiol. vol. 97, pp. 1044–1050 (1991); B. Miki et al., “Transformation of Brassica napus canola cultivars with Arabidopsis thaliana Acetohydroxyacid Synthase Genes and Analysis of Herbicide Resistance,” Theor. Appl. Genet., vol. 80, pp. 449–458 (1990); P. Wiersma et al., “Isolation, Expression and Phylogenetic Inheritance of an Acetolactate Synthase Gene from Brassica napus,” Mol. Gen. Genet., vol. 219, pp. 413–420 (1989); and J. Odell et al., “Comparison of Increased Expression of Wild-Type and Herbicide-Resistant Acetolactate Synthase Genes in Transgenic Plants, and Indication of Postranscriptional Limitation on Enzyme Activity,” Plant Physiol., vol. 94, pp. 1647–1654 (1990).
S. Sebastian et al., “Soybean Mutants with Increased Tolerance for Sulfonylurea Herbicides,” Crop. Sci., vol. 27, pp. 948–952 (1987) discloses soybean mutants resistant to sulfonylurea herbicides. See also U.S. Pat. No. 5,084,082.
K. Shimamoto et al., “Fertile Transgenic Rice Plants Regenerated from Transformed Protoplasts,” Nature, vol. 338, pp. 274–276 (1989) discloses a genetic transformation protocol in which electroporation of protoplasts was used to transform a gene encoding β-glucuronidase into rice.
T. Terakawa et al., “Rice Mutant Resistant to the Herbicide Bensulfuron Methyl (BSM) by in vitro Selection,” Japan. J. Breed., vol. 42, pp. 267–275 (1992) discloses a rice mutant resistant to a sulfonylurea herbicide, derived by selective pressure on callus tissue culture. Resistance was attributed to a mutant AHAS enzyme.
Following are publications by the inventor (or the inventor and other authors) concerning research on herbicide-resistant rice varieties. These publications are T. Croughan et al., “Rice and Wheat Improvement through Biotechnology,” 84th Annual Research Report, Rice Research Station, 1992, pp. 100–103 (1993); T. Croughan et al., “Rice and Wheat Improvement through Biotechnology,” 85th Annual Research Report, Rice Research Station, 1993, pp. 116–156 (1994); T. Croughan, “Application of Tissue Culture Techniques to the Development of Herbicide Resistant Rice,” Louisiana Agriculture, vol. 37, no. 3, pp. 25–26 (1994); T. Croughan et al., “Rice Improvement through Biotechnology,” 86th Annual Research Report, Rice Research Station, 1994, pp. 461–482 (1995); T. Croughan et al., “Assessment of Imidazolinone-Resistant Rice,” 87th Annual Research Report, Rice Research Station, 1994, pp. 491–525 (September 1996); T. Croughan et al., “IMI-Rice Evaluations,” 88th Annual Research Report, Rice Research Station, 1996, pp. 603–629 (September 1997); T. Croughan et al., “Imidazolinone-Resistant Rice,” 89th Annual Research Report, Rice Research Station, 1997, p. 464 (September 1998); T. Croughan et al., “Rice and Wheat Improvement through Biotechnology,” USDA CRIS Report Accession No. 0150120 (for Fiscal Year 1994—actual publication date currently unknown); T. Croughan et al., “Improvement of Lysine Content and Herbicide Resistance in Rice through Biotechnology,” USDA CRIS Report Accession No. 0168634 (for Fiscal Year 1997—actual publication date currently unknown); T. Croughan, “Herbicide Resistant Rice,” Proc. 25th Rice Tech. Work. Groups, p. 44 (1994); T. Croughan et al., “Applications of Biotechnology to Rice Improvement,” Proc. 25th Rice Tech. Work. Groups, pp. 62–63 (1994); T. Croughan, “Production of Rice Resistant to AHAS-Inhibiting Herbicides,” Congress on Cell and Tissue Culture, Tissue Culture Association, In Vitro, vol. 30A, p. 60, Abstract P-1009 (Jun. 4–7, 1994). (Note that the Annual Research Reports of the Rice Research Station are published in the year after the calendar year for which activities are reported. For example, the 84th Annual Research Report, Rice Research Station, 1992, summarizing research conducted in 1992, was published in 1993.) The reports in the 87th and 88th Annual Research Report, Rice Research Station (published September 1996 and September 1997, respectively) mention the breeding line 93AS3510 in tables giving data on certain herbicide resistance trials. These reports gave no information on how the breeding line was developed. The breeding line was not publicly available at the times these reports were published. The breeding line 93AS3510 is the same as the ATCC 97523 rice that is described in greater detail in the present inventor's later-published international application WO 97/41218 (1997) and U.S. Pat. Nos. 5,736,629, 5,773,704, 5,952,553, and 6,274,796.
See also E. Webster et al., “Weed Control Systems for Imi-Rice,” p. 33 in Program of the 27th Rice Technical Working Group Meeting (March 1998); L. Hipple et al., “AHAS Characterization of Imidazolinone Resistant Rice,” pp. 45–46 in Program of the 27th Rice Technical Working Group Meeting (March 1998); W. Rice et al., “Delayed Flood for Rice Water Weevil Control using Herbicide Resistant Germplasm,” p. 61 in Program of the 27th Rice Technical Working Group Meeting (March 1998); E. Webster et al., “Weed Control Systems for Imidazolinone-Rice,” p. 215 in Proceedings of the 27th Rice Technical Working Group Meeting (1999); L. Hipple et al., “AHAS Characterization of Imidazolinone Resistant Rice,” pp. 68–69 in Proceedings of the 27th Rice Technical Working Group Meeting (1999); and W. Rice et al., “Delayed Flood for Rice Water Weevil Control using Herbicide Resistant Germplasm,” p. 134 in Proceedings of the 27th Rice Technical Working Group Meeting (1999).
The present inventor's U.S. Pat. No. 5,545,822 discloses a line of rice plants having a metabolically-based resistance to herbicides that interfere with the plant enzyme acetohydroxyacid synthase; i.e., the herbicide resistance of these rice plants was not due to a resistant AHAS enzyme. (See published international application WO 97/41218, pages 6–9.) See also the present inventor's U.S. Pat. No. 5,773,703.
The present inventor's published international application WO 97/41218 discloses one line of rice plants having a mutant AHAS enzyme that is resistant to herbicides that interfere with the wild-type plant enzyme acetohydroxyacid synthase. This line of rice plants was developed by exposing rice seeds to the mutagen methanesulfonic acid ethyl ester (EMS), and screening millions of progeny for herbicide resistance. See also the present inventor's U.S. Pat. Nos. 5,736,629, 5,773,704, 5,952,553, and 6,274,796.
U.S. Pat. No. 4,443,971 discloses a method for preparing herbicide tolerant plants by tissue culture in the presence of herbicide. U.S. Pat. No. 4,774,381 discloses sulfonylurea (sulfonamide) herbicide-resistant tobacco plants prepared in such a manner.
U.S. Pat. No. 5,773,702 discloses sugar beets with a resistant mutant AHAS enzyme, derived from cell cultures grown in the presence of herbicide.
U.S. Pat. No. 5,633,437 discloses a herbicide resistant AHAS enzyme and gene isolated from cockleburs.
U.S. Pat. No. 5,767,361 discloses a mutant, resistant AHAS enzyme from maize. The definitions of the U.S. Pat. No. 5,767,361 patent are incorporated into the present disclosure by reference, to the extent that those definitions are not inconsistent with the present disclosure. See also U.S. Pat. No. 5,731,180 and European Patent Application 0 525 384 A2.
U.S. Pat. No. 5,605,011; European Patent Application 0 257 993 A2; and European Patent Application 0 730 030 A1 disclose resistant acetolactate synthase (ALS, another name for AHAS) enzymes based on enzymes derived from callus culture of tobacco cells in the presence of herbicide, from spontaneous mutations of the ALS gene in yeast; EMS-induced mutations in Arabidopsis seeds; certain modifications of those enzymes; and the transformation of various plants with genes encoding the resistant enzymes. These patents disclose several techniques for modifying AHAS genes to produce herbicide-resistant AHAS enzymes, and for transforming plants with those genes.
U.S. Patent Re 35,661 (a reissue of U.S. Pat. No. 5,198,599) discloses lettuce plants with enhanced resistance to herbicides that target the enzyme acetolactate synthase. The initial source of herbicide resistance was a prickly lettuce weed infestation in a grower's field, an infestation that was not controlled with commercial sulfonylurea herbicides.